1. Field of the Invention
The present invention relates to a cell exchange apparatus for high-speed exchange of various information such as sound, data and picture information in the form of blocks which are called cells.
2. Description of the Prior Art
A switching network, which is called Banyan-network, has heretofore been known as one example of the system in which data is divided into cells and the header information of each cell is directly referred to by hardware means to thereby perform high-speed switching.
FIG. 7 is a block diagram of a conventional cell exchange apparatus which is disclosed, for example in IEEE Journal on Selected Areas in Communications Vol. SAC-4, No. 8, November 1986, pp. 1373-1380. In the figure, reference numerals 1a and 1b denote cells each comprising a header section and a data section, 2 input ports to which cells 1 (1 denoting generally the cells 1a and 1b) are inputted, 3 a cell exchange apparatus that performs switching of cells 1 which are inputted to input ports 2, 4 2.times.2 unit switches serving as space switches which constitute the cell exchange apparatus 3, and 5 output ports of the cell exchange apparatus 3.
It should be noted that the above-described literature shows a Banyan-network in which the header information of each cell 1 is directly referred to by hardware means to thereby perform high-speed switching, and it uses the term "packet" in place of the term "cell". However, "cell" and "packet" express the same thing in the sense that multimedia information is divided into blocks and a header that includes destination information is given to each block. However, "packet" and "cell" are generally different from each other in that the length of each block of packet is handled as being variable, whereas that of cell is handled as being fixed according to the provisions of international standards. Since the term "cell" is used in asynchronous transmission mode (ATM) communication wherein high-speed transmission and exchange of data are performed, the term "cell" will be employed in place of "packet" in the following description of the prior art.
In operation, each of the 2.times.2 unit switches 4 that comprise the cell exchange apparatus 3, shown in FIG. 7, selects an output port 5 in accordance with the state of the corresponding bit in a bit string that constitutes the header section of a cell 1. For example, each of the unit switches 4 that are arrayed in the first row from the left as viewed in FIG. 7 performs a switching operation such that, if the top bit in the header section of a cell 1 is "0", the unit switch 4 connects the input port thereof to the upper output port thereof, whereas, if the top bit is "1", it connects the input port to the lower output port. The unit switches 4 that are arrayed in the second row from the left similarly perform a switching operation in accordance with the state of the second bit in the header section of the cell 1. If such unit switches 4 are interconnected as shown in FIG. 7 and the number of a desired output port 5 in the final stage is expressed as a binary number and put to the header section of a cell 1, the cell 1 arrives at the desired output port 5 no matter from which input port 2 it is inputted.
The conventional cell exchange apparatus having the above-described arrangement suffers, however, from the following problems. If cells 1a and 1b which are directed to the same output port 5 are simultaneously inputted to input ports 2, blocking (collision) occurs. In order to solve this problem, a system wherein a buffer memory is provided in the input section or inside of each unit switch 4 has been proposed. With this prior art system, however, the buffer memory is likely to be blocked when cells 1 which are directed to one output port 5 are concentrated therein.